1. Field of the Invention
This invention relates to methods of powder formation and thin film deposition from reagents contained in liquid or liquid-like fluid solutions, whereby the fluid solution, near its supercritical point temperature, is released into a region of lower pressure causing a superior, very fine atomization or vaporization of the solution. Gasses are entrained or fed into the dispersed solution and rapidly flow into a flame or plasma torch. The reagents react and form either: 1) powders which are collected; or 2) a coating from the vapor phase onto a substrate positioned in the resulting gases and vapors. Release of the near supercritical point temperature fluid causes dispersion and expansion resulting in a very fine nebulization of the solution, which yields improved powder and film quality, deposition rates and increases the number of possible usable precursors.
2. Background of the Invention
Chemical vapor processing has been used extensively for the production of powders and coatings. Chemical vapor deposition ("CVD") is the term used when coatings onto a substrate are formed. CVD production of coatings is widespread. Many of these coating are only nanometers thick and smooth to less than 5% percent of coating thickness. Reaction and agglomeration of the reacted vapor material in the gas stream forms powders which can be commercially useful. In fact, nanopowders are required in the formation of nanomaterials which have different properties from those of bulk materials. These materials' properties can be tailored by controlling the cluster size of the nanopowder. Similarly, coatings of less than 50 mn can have properties which are different from thicker films, and the properties change further as the coating thins.
It is desirable to form such powders and coatings at low production and capitalization costs and with simple production processes. However, for many materials there is a very limited selection of available precursors which can be vaporized and used for traditional CVD. Being able to form coatings in the open atmosphere tremendously eases substrate handling and flow through the coating process. In addition to thin films low cost quality thick coatings and bulk materials are also desirable.
Combustion chemical vapor deposition ("CCVD"), a recently invented CVD technique, allows for open atmosphere deposition of thin films. The CCVD process offers several advantages over other thin-film technologies, including traditional CVD. The key advantage of CCVD is its ability to deposit films in the open atmosphere without any costly furnace, vacuum, or reaction chamber. As a result, the initial system capitalization requirement can be reduced up to 90% compared to a vacuum based system. Instead of a specialized environment, which is required by other technologies, a combustion flame provides the necessary environment for the deposition of elemental constituents from solution, vapor, or gas sources. The precursors are generally dissolved in a solvent that also acts as the combustible fuel. Depositions can be performed at atmospheric pressure and temperature within an exhaust hood, outdoors, or within a chamber for control of the surrounding gasses or pressure.
Since CCVD generally uses solutions, a significant advantage of this technology is that it allows rapid and simple changes in dopants and stoichiometries which eases deposition of complex films. In contrast to conventional CVD, where the precursor vapor pressure is a concern which dictates expensive high vapor pressure precursors, the CCVD technique generally uses inexpensive, soluble precursors. In addition, precursor vapor pressures do not play a role in CCVD because the dissolution process provides the energy for the creation of the necessary ionic constituents. In general, the precursor materials used for traditional CVD depositions are between 10 and 1000 times more expensive than those which can be used in CCVD processing. By adjusting solution concentrations and constituents, a wide range of stoichiometries can be deposited quickly and easily. Additionally, the CCVD process allows both chemical composition and physical structure of the deposited film to be tailored to the requirements of the specific application.
Unlike CVD, the CCVD process is not confined to an expensive, inflexible, low-pressure reaction chamber. Therefore, the deposition flame, or bank of flames, can be moved across the substrate to easily coat large and/or complex surface areas. Because the CCVD process is not limited to specialized environments, the user can continuously feed materials into the coating area without disruption, thereby permitting batch processing. Moreover, the user can limit deposition to specific areas of a substrate by simply controlling the dwell time of the flame(s) on those areas. Finally, the CCVD technology generally uses halogen free chemical precursors having significantly reduced negative environmental impact compared to conventional CVD, resulting in more benign by-products.
Numerous materials have been deposited via CCVD technology with the combustion of a premixed precursor solution as the sole heat source. This inexpensive and flexible film deposition technique permits broader use of thin film technology. The CCVD process has much of the same flexibility as thermal spraying, yet creates quality, conformal films like those associated with CVD. Traditional CVD often requires months of effort to successfully deposit a material. With CCVD processing, a desired phase can be deposited in a few days and at a fraction of the cost of traditional CVD.
By providing these coating capabilities inexpensively, the CCVD process can broaden the commercial opportunity for thin films, including use in tribological, thermal protective, wear, space environment protective, optic, electronic, structural and chemical resistant applications. Thus, government and commercial users can benefit from the advantages of thin films over thick films, including their high adhesion to the substrate, controlled microstructure, greater flexibility, reduced raw material consumption and reduced effect on the operating characteristics and/or dimensions of the coated system.
Ichinose, H., Shiwa, Y., and Nagano, M., Synthesis of BaTiO.sub.3 /LaNiO.sub.3 and PbTiO.sub.3 /LaNiO.sub.3 Thin Films by Spray Combustion Flame Technique, Jpn. J. Appl. Phys., Vol. 33, 1, 10 p. 5903-6 (1994) and Ichinose, H., Shiwa, Y., and Nagano, M., Deposition of LaMO.sub.3 (M.dbd.Ni, Co, Cr, Al)--Oriented Films by Spray Combustion Flame Technique, Jpn. J. Appl. Phys., Vol. 33, 1, 10 p. 5907-10 (1994) used CCVD processing, which they termed spray combustion flame technique, by ultrasonically atomizing a precursor containing solution, and then feeding the resulting nebulized solution suspended in argon carrier gas into a propane combustion flame. However, these atomization techniques cannot reach the highly desirable submicron capabilities which are important to obtaining improved coating and powder formation.
U.S. Pat. No. 4,582,731 (the "'731 patent") discloses the use of a supercritical fluid molecular spray for the deposition of films. However, the '731 patent is for physical vapor deposition (PVD), which differs from the independently recognized field of CVD by having no chemical reagents and normally being operated at high vacuum. Additionally, no flame or plasma torch is used in this method, and only supercritical fluid solutions are considered. Chemical reagents are beneficial because of there physical properties, including higher solubility. The flame and plasma torch enable coatings in the open atmosphere with no additional heat source. The '731 deposition material, however, does not start from a reagent, and thus will not react at supercritical conditions.
U.S. Pat. No. 4,970,093 (the "'093 patent") discloses the use of supercritical fluids and CVD for the deposition of films. Work related to the '083 patent is described in B. M. Hybertson, B. N. Hansen, R. M. Barkley and R. E. Sievers, Supercritical Fluid Transport-Chemical Deposition of Films, Chem. Mater., 4, 1992, p. 749-752 and Hybertson et al and B. N. Hansen, B. M. Hybertson, R. M. Barkley and R. E. Sievers, Deposition of Palladium Films by a Novel Supercritical Fluid Transport-Chemical Deposition Process produce, Mat. Res. Bull., 26, 1991, p. 1 127-33. The '093 patent is for traditional CVD without a flame or plasma torch and does not consider open atmosphere capable techniques such as CCVD, which has the associated advantages discussed above. Additionally, only supercritical fluid solutions are considered; liquid solutions near the supercritical point are not addressed. All of the precursors of the '093 patent are carried in the supercritical solution which can limit the usable precursors due to reactivity and solubility in supercritical fluids.
B. M. Merkle, R. N. Kniseley, F. A. Smith and I. E. Anderson, Superconducting YBaCuO Particulate produced by Total Consumption Burner Process produce, Mat. Sci. Eng., A124, p.31-38 (1990), J. McHale et al., Preparation of High-Tc Oxide Films via Flaming Solvent Spray, J. Supercond. 5 (6), p.511 (1992), and M. Koguchi et al., Preparation of YBa.sub.2 Cu.sub.3 O.sub.x Thin Film by Flame Pyrolysis, Jpn. J. Appl. Phys. 29 (1), p.L33 (1990) describe the use of a flame to deposit films in what was termed a "spray pyrolysis" technique. Both Merkle et al. and McHale et al. deposited YBa.sub.2 Cu.sub.3 O.sub.x from a combusted sprayed solution onto substrates, but the deposition conditions resulted in low quality pyrolysis and particulate type coatings. Koguchi et al. atomized a 0.03 mol/l aqueous solution and transported the resulting mist into a H.sub.2 --O.sub.2 flame and deposited a 10 .mu.m thick coating in 10 minutes on a yttria stabilized zirconia (YSZ) substrate heated by the flame with much of the sprayed material being lost in transport due to the method used. The temperature, measured at the back of the substrate, reached a maximum of 940.degree. C. However, the flame side of the substrate is generally expected to be 100.degree. C. to 300.degree. C. higher in temperature than the back which would be in the melting range of YBa.sub.2 Cu.sub.3 O.sub.x. The resulting Koguchi et al. film had a strong c-axis preferred orientation and, after a 850.degree. C. oxygen anneal for eight hours, the film showed zero resistivity at 90.degree. K. Koguchi et al. termed their method "flame pyrolysis," and were probably depositing at temperatures near the melting point of YBa.sub.2 Cu.sub.3 O.sub.x. The solution concentrations and deposition rates were higher than those useful in CCVD processing. Therefore, there exists a need for a coating method which achieves excellent results at below the coating materials' melting point. The present invention fulfills this need because the finer atomization of the near supercritical fluid improves film quality by enabling the formation of vapor deposited films at lower deposition temperatures.
McHale et al. successfully produced 75 to 100 .mu.m thick films of YBa.sub.2 Cu.sub.3 O.sub.x and Bil.sub.1.7 Pb.sub.0.3 Ca.sub.2 Sr.sub.2 Cu.sub.3 O.sub.10 by combusting a sprayed solution of nitrates dissolved in liquid ammonia in a N.sub.2 O gas stream, and by combusting nitrates dissolved in either ethanol or ethylene glycol in an oxygen gas stream. The results suggest the films were particulate and not phase pure. The YBa.sub.2 Cu.sub.3 O.sub.x coatings had to be annealed at 940.degree. C. for 24 hours and the Bi.sub.1.7 Pb.sub.0.3 Ca.sub.2 Sr.sub.2 Cu.sub.3 O.sub.10 coatings heat treated at 800.degree. C. for 10 hours and then at 860.degree. C. for 10 hours to yield the desired material. Even after oxygen annealing, zero resistivity could never be obtained at temperatures above 76.degree. K. The solution concentrations used were not reported, but the deposition rates were excessively high. In both Koguchi's and McHale's methods, the reported solution and resulting film stoichiometries were identical. Conversely, in CVD and in the present invention, the solution stoichiometry may differ from the desired film stoichiometry. Additionally the resulting droplet size of sprayed solutions was excessively large and the vapor pressure too low for effective vapor deposition.
A nebulized solution of precursors has been used with a plasma torch in a process termed "spray inductively coupled plasma" ("spray-ICP" or "ICP"). See M. Kagawa, M. Kikuchi, R. Ohno and T. Nagae, J. Amer. Ceram. Soc., 64, 1981, C7. In spray-ICP, a reactant containing solution is atomized into fine droplets of 1-2 mm in diameter which are then carried into an ICP chamber. This can be regarded as a plasma CVD process, different from flame pyrolysis. See M. Suzuki, M. Kagawa, Y. Syono, T. Hirai and K. Watanabe, J. Materials Sci., 26, 1991, p.5929-5932. Thin films of the oxides of Ce, La, Y, Pr, Nd, Sm, Cr, Ni, Ti, Zr, La--Sr--Cu, Sr--Ti, Zn--Cr, La--Cr, and Bi--Pb--Sr--Ca--Cu have successfully been deposited using this technique. See M. Suzuki, M. Kagawa, Y. Syono and T. Hirai, Thin films of Chromium Oxide Compounds Formed by the Spray-ICP Technique, J. Crystal Growth, 99, 1990, p.611-615 and M. Suzuki, M. Kagawa, Y. Syono and T. Hirai, Thin films of Rare-Earth (Y, La, Ce. Pr, Nd, Sm) Oxides Formed by the Spray-ICP Technique, J. Crystal Growth, 112, 1991, p.621-627. Holding the substrate at an appropriate distance from the plasma was crucial to synthesizing dense films. The range of desired deposition distances from the plasma source was small due to the rapid temperature drop of the gases. CVD type coatings were achieved using ultrasonically atomized 0.5-1.0 M solutions of metal-nitrates in water which were fed into the ICP at 6-20 ml/h using Ar flowing at 1.3-1.4 l/min. Only oxides were deposited and liquid or liquid-like solutions near the supercritical temperature were not used. The use of near supercritical atomization with ICP was not considered in this broad review of ICP nebulization techniques. See T. R. Smith and M. B. Denton, Evaluation of Current Nebulizers and Nebulizer Characterization Techniques, Appl. Spectroscopy, 44, 1990, p.21-4.
Therefore, it is highly desirable to be able to form nanopowders and coatings at low production and capitalization costs and with simple production processes. It is also desirable to be able to form coatings in the open atmosphere without any costly furnace, vacuum, or reaction chamber. It is further highly desirable to provide a coating process which provides for a high adhesion to the substrate, controlled microstructure, flexibility, reduced raw material consumption and reduced effect on the operating characteristics and/or dimensions of the coated system while being able to retain highly desirable submicron capabilities which are important to obtaining improved coating and powder formation. Moreover, it is highly desirable to provide a process which uses solutions near their supercritical point, and, therefore, achieves excellent results at below the coating materials' melting point.